Insights into Pre-eruptive Magma Storage, Differentiation, and Eruption Triggering from the 36.5 Ma Cueva Tuff, Organ Caldera, New Mexico

Abstract:

Caldera-forming eruptions can eject large volumes of highly evolved magma and typically require a “trigger” (e.g., injection or underplating of hotter magma, tectonic triggers, chamber overpressure) to mobilize these large, viscous melt bodies. Here we investigate pre-eruptive storage conditions, differentiation, and eventual eruption of the Cueva Tuff magma, the first of three caldera-forming eruptions from Organ Caldera in southern New Mexico. Analyses of quartz-hosted melt inclusions from the middle of the tuff sequence reveal that the magma was compositionally similar to topaz rhyolites erupted across the western US during this time period, with high volatile contents (<6.8 wt% H₂O, <0.5 wt% F) and enrichments in lithophile elements. Melt inclusion H₂O and CO₂ (<200 ppm) contents indicate isobaric storage and crystallization at ~275 MPa (~10 km depth). Major and trace element compositions indicate are heterogeneous and consistent with ~30% crystallization of a non-stratified magma body. Mineralogy of the tuffs indicates crystallization of quartz > sanidine (~Ab₅₅Or₄₅) and minor biotite, consistent with major and trace element variations in the melt inclusions.

What triggered the eruption? Increasingly, high-Ba rims on sanidine have been found in large-volume tuffs, including in calderas across southern New Mexico; these rims are evidence of magma injection or underplating that re-melted a cumulate pile and likely triggered the eruption (e.g., Wolff et al., 2015 doi:10.1016/j.lithos.2015.09.002). However, Cueva quartz and sanidine lack resorption textures and sanidine do not have high-Ba rims. Instead, we suggest that the ≥30% crystallization of the volatile-rich Cueva magma was an internal trigger that contributed to eruption. Using MELTS modeling, we found overpressures of ~30 MPa are achieved during 30% fractional crystallization. These overpressures are minima, as we cannot constrain previously exsolved volatiles in the magma body, but they were likely sufficient to prime the system for eruption.